Ylide carbanion structures, Wittig reaction

The stereoselectivity of the Wittig reaction depends on the structure of the ylide. Ylides can be divided into two types Stabilized ylides have a group, such as a carbonyl group, that can share the carbanion s negative charge unstabilized ylides do not have such a group. 

As indicated vide supra), the classical mechanism for the Wittig reaction involves the initial nucleophilic addition of the ylide carbanion (resonance structures 1 and 2) to the electrophilic carbon of the carbonyl in 3 to afford betaine intermediate 4. Rotation about the central C-C bond provides for ring closure to 1,2-oxaphosphetane 5. These species are thermally unstable and readily decompose, via a concerted electrocyclic process, to generate the corresponding phosphine oxide 8 and the ( )-alkene 6 or (Z)-alkene 7. 

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